Methods and apparatuses for processing images and in particular images of biological particles in a fluid sample are well known in the art. See, for example, U.S. Pat. Nos. 4,667,335 and 4,612,614 assigned to the present assignee. In those prior art references, the apparatus uses a computer having a software program to determine various characteristics of particles and in particular biological particles under examination in an imaging signal. Thus, for example, various characteristics such as color, size, concentration, etc. can be determined by the processor. The processor then uses an algorithm incorporated in its software, to identify or classify each particle based upon the characteristics determined.
In the prior art, a portion of a fluid sample is analyzed. The number of particles determined in that portion is counted. Another portion is then analyzed, and the number of particles determined from that portion is added to the total counted. This process continues until a maximum amount of time for the fluid sample flowing through a flow cell at a substantially constant volume rate is reached or a maximum number of particles is counted. The former corresponds to a maximum volume of the fluid sample being analyzed, whereas the latter corresponds to a maximum concentration.
The problem with this prior art method is that if there are a plurality of different types of particles in a fluid sample, and almost always for biological fluid samples, there are, certain types of particles occurring in low concentration, may not be observed by this method. This is because, if the maximum volume is set low, to encourage greater throughput, the method of prior art invariably causes the method to stop after the larger concentration type of particles is observed.